1. Field of the Invention
The present invention relates to an image-recording device, and more particularly to an image-recording device that conveys an image recording sheet and performs image recording on a surface of the sheet.
2. Description of the Related Art
Conventionally, in an image-recording device such as, for example, an inkjet recording device, image recording has been carried out by conveying paper for recording (a recording sheet) in a certain direction while discharging ink drops onto the recording paper in accordance with image information. That is, an inkjet recording head is operated to move in a direction intersecting the conveyance direction of the recording paper while discharging the ink drops (main scanning), and is repeatedly reciprocated as the recording paper is conveyed (sub-scanning). Thus, an image is recorded on the recording paper. Accordingly, in order to record images at higher image quality, improvements in accuracy of conveyance of the recording paper are required.
Many ordinary inkjet recording devices and the like have structures in which the recording paper is conveyed by supply rollers, which are disposed at an upstream side of an image recording region, and ejection rollers, which are disposed at a downstream side. In an inkjet recording device having such a structure, in order that the recording paper can be conveyed with high accuracy throughout the entire conveyance direction range of the recording paper, a speed of rotation of the supply rollers is set to a prescribed value corresponding to a conveyance speed of the recording paper. A speed of rotation of the ejection rollers may be set to be slightly faster than the speed of rotation of the supply rollers. In such a constitution, the recording paper is conveyed at a certain conveyance speed by the supply rollers. The ejection rollers rotate so as to slip a little with respect to the recording paper, and the recording paper is stretched between the ejection rollers and the supply rollers while being conveyed. Consequently, a separation between the inkjet recording head and the recording paper is kept stable. Hence, a reduction in image quality that could be caused by variations in this separation can be avoided.
However, in an inkjet recording device having the structure described above, when a back end vicinity portion of the recording paper is released from the supply rollers during the conveyance of the recording paper, the recording paper is subsequently conveyed only by the ejection rollers. Here, because the ejection rollers rotate more quickly than the supply rollers, the conveyance speed of the recording paper becomes larger. Thus, for example, the intervals between the recording lines may become larger and “white ground” portions may result.
As a solution to this kind of problem, Japanese Patent No. 2,810,476, for example, describes a control method for transporting paper in a recording device. A sheet transport speed of a pair of rotating bodies at a recording sheet feed-in side of a printing section is set to V1, a sheet transport speed of a pair of rotating bodies at a recording sheet feed-out side of the printing section is set to V2, and V2>V1. While the sheet is nipped by the rotating body pairs at both the feed-in side and the feed-out side or by only the rotating body pair at the feed-in side, the sheet is transported at the speed V1. But when a rear end of the paper has separated from the rotating body pair at the feed-in side, the sheet transport speed of the rotating body pair at the recording sheet feed-out side is (immediately) adjusted from V2 to V1.
However, to implement this control method, it is necessary to provide, for example, a sensor for detecting when the recording sheet has passed through the rotating body pair at the feed-in side and a pulse generator or the like for efficiently applying input pulses to motors that drive each of the rotating body pairs. Hence, the number of components increases, and this leads to increased costs.
Accordingly, Japanese Patent Application Laid-Open (JP-A) No. 11-91177 describes an image-recording device constituted so as to alter control of image recording when a recording sheet is being conveyed by only one of an upstream side conveying means and a downstream side conveying means. For example, when the recording sheet is being conveyed by only the downstream side conveying means, a recording interval may be reduced. Also, conveyance amounts by the downstream side conveying means are correspondingly reduced. In such a device, control is performed suitably for a front end and a back end of the recording sheet, and thus printing quality is improved.
However, in this structure, when the conveyance amounts of the recording sheet are decreased to such an extent that a deterioration of print quality does not occur, a number of scanning passes by a carriage, to which an inkhead is mounted, is relatively increased by the same proportion. Consequently, when image recording is to be applied to the whole of the recording sheet, recording duration is lengthened.